1. Field of the Invention
The present invention relates to communications systems, and more particularly to a method for controlling the transmission of polling a protocol data unit (PDU) in a wireless communications system.
2. Background of the Related Art
A universal mobile telecommunications system (UMTS) is a third generation mobile communication system that has evolved from a standard known as Global System for Mobile communications (GSM). This standard is a European standard which aims to provide an improved mobile communication service based on a GSM core network and wideband code division multiple access (W-CDMA) technology. In December, 1998, the ETSI of Europe, the ARIB/TTC of Japan, the T1 of the United States, and the TTA of Korea formed a Third Generation Partnership Project (3GPP) for the purpose of creating the specification for standardizing the UMTS.
The work towards standardizing the UMTS performed by the 3GPP has resulted in the formation of five technical specification groups (TSG), each of which is directed to forming network elements having independent operations. More specifically, each TSG develops, approves, and manages a standard specification in a related region. Among them, a radio access network (RAN) group (TSG-RAN) develops a specification for the function, data desired, and interface of a UMTS terrestrial radio access network (UTRAN), which is a new RAN for supporting a W-CDMA access technology in the UMTS.
The TSG-RAN group includes a plenary group and four working groups. Working group 1 (WG1) develops a specification for a physical layer (a first layer). Working group 2 (WG2) specifies the functions of a data link layer (a second layer) and a network layer (a third layer). Working group 3 (WG3) defines a specification for an interface among a base station in the UTRAN, a radio network controller (RNC), and a core network. And, Working group 4 (WG4) discusses requirements desired for a radio link performance and data desired for radio resource management.
FIG. 1 shows the structure of a radio access interface protocol used between a terminal operating based on a 3GPP RAN specification and a UTRAN.
When viewed horizontally, the radio access interface protocol includes a physical layer (PHY), a data link layer, and a network layer; and when viewed vertically the protocol is divided into a control plane (C-plane) for transmitting a control signal and a user plane for transmitting data information. The user plane is a region to which traffic information of a user such as voice or an IP packet is transmitted. The control plane is a region to which control information such as an interface of a network or maintenance and management of a call is transmitted.
Also, the protocol layers may be divided into a first layer (L1), a second layer (L2), and a third layer (L3) based on three lower layers of an open system interconnection (OSI) standard model well known in a communication system.
The first layer (L1) operates as a physical layer (PHY) for a radio interface and, according to related technology, is connected to an upper medium access control (MAC) layer through one or more transport channels. The physical layer transmits data delivered to the physical layer (PHY) through a transport channel to a receiver using various suitable coding and modulating methods.
The second layer (L2) operates as a data link layer and lets various terminals share the radio resources of a W-CDMA network. The second layer (L2) is divided into the MAC layer, a radio link control (RLC) layer, a packet data convergence protocol (PDCP) layer, and a broadcast/multicast control (BMC) layer.
Among them, the RLC layer forms an appropriate protocol data unit (PDU) suitable for transmission by the segmentation and concatenation functions of a service data unit (SDU) received from an upper layer. The RLC layer also performs an automatic repeat request (ARQ) function by which a PDU lost during transmission is re-transmitted. The RLC layer operates in three modes, a transparent mode (TM), an unacknowledged mode (UM), and an acknowledged mode (AM). The mode selected depends upon the method used to process the RLC SDU received from the upper layer. Also, an RLC buffet stores the received SDUs or the RLC buffers for storing the PDUs received from the upper layer exists in the RLC layer.
In addition, the packet data convergence protocol (PDCP) layer is an upper layer of the RLC layer which allows data items to be transmitted through a network protocol such as the IP.v4 or the IP.v6. A header compression technique for compressing and transmitting the header information in a packet can be used for effective transmission of the IP packet.
The RLC layer will now be described in more detail.
As previously indicated, the RLC layer operates in three modes: TM, UM, and AM. The AM mode will now be described.
One of the most significant characteristics of AM mode operation is its ability to support the re-transmission of a PDU when the PDU is not successfully transmitted or received. More specifically, when the transmitter RLC layer transmits a PDU, the receiver determines whether each PDU is received and then generates status information indicating the result. The receiver then sends the status information back to inform the transmitter as to whether the PDU was received. When the transmitter receives the status information from the receiver indicating that the PDU was not received, the PDU is re-transmitted to the receiver.
A process for transmitting the PDU in the transmitting RLC will be described with reference to FIG. 2. FIG. 2 shows a structure of a RLC transmitter 100 which transmits PDUs to a receiver.
As shown, when a PDU generator 101 receives an SDU from an upper layer, the PDU generator segments or concatenates the SDU in order to make the SDU a uniform size of a PDU. A PDU may be generated by adding an RLC header to each segment, and a sequence number may be included in the header. The PDU may be classified based on its sequence number.
PDUs generated in this way are stored in both a transmission buffer 102 and a re-transmission buffer 104. The RLC transmitter sends the PDUs stored in transmission buffer 102 to a lower layer based on a number requested by the lower layer every transmission time interval (TTI). At this time, a polling bit setting unit 104 determines whether to set a polling bit requesting the receiver to send status information for a specific PDU among the transmitted PDUs. The PDU in which the polling bit is to be set varies according to a polling trigger.
Thereafter, the PDUs sent to the lower layer are transmitted to the receiver through a radio interface. In the receiver, a RLC forms SDUs using information in the headers of the PDUs, and SDUs are then delivered to the receiver upper layer.
When the polling bit is set in one PDU among the received PDUs, the receiver RLC checks whether the PDUs are correctly received and transmits status information to the transmitter RLC. Then, the transmitter RLC deletes successfully transmitted PDUs from re-transmission buffer 102. PDUs that are not successfully transmitted, as determined by negative status information, are sent to the transmission buffer and are re-transmitted.
At this time, only PDUs which receive a negative acknowledgment are re-transmitted. The re-transmitted PDUs are left in the re-transmission buffer until the transmission is determined to be successful. Re-transmitted PDUs may be given priority over first-transmitted PDUs, and it is possible to set a polling bit in the re-transmitted PDU.
In the RLCs of transmitting and receiving sides, a transmission window and reception window are respectively used to transmit and receive PDUs. In general, the size of the transmission window is the same as the size of the reception window. The transmission window has a size which corresponds to a predetermined maximum number of PDUs that can be transmitted. PDUs within the range of the window are transmitted from a transmission buffer, subsequent PDUs are loaded into the buffer. The reception window in the receiver receives only PDUs that lie within a valid range. More specifically, the receiver receives only PDUs having transmission sequence numbers that lie within the limits of the reception window. PDUs received beyond the range of the reception window are discarded as soon as the PDUs are received.
Generally, a polling means that the transmitter requests status information from the receiver. When the receiver receives a polling request from the transmitter, the receiver must check the reception state of PDUs received up to that point. The receiver then sends information concerning this reception state to the transmitter.
Therefore, for polling, the transmitter sets a polling bit in a PDU before transmission. When the PDU containing the polling bit is received, the receiver checks the state of a reception buffer with respect to this and previously received PDUs, and then informs the transmitter of information concerning whether each PDU up to that point was successfully received.
Because transmitting status information wastes radio resources, transmission of status information must be controlled by an appropriate method. That is, the transmitter must set the polling bit only for a PDU which satisfies a certain rule, without requesting the status information in every PDU. Such a rule is known as a polling trigger.
As described above, conventionally, the polling bit is set in the transmitted PDU using a fixed polling trigger. Therefore, in spite of the situation where the status information is not necessarily required by the receiver, that is, a situation where the PDUs can be transmitted without polling in the wireless environment, the transmitting RLC requires polling by the receiver according to the fixed polling trigger, causing a waste of radio resources and data transmission delay.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.